A detailed re-description and revised diagnosis is given for Macroplata tenuiceps Swinton, 1930, a plesiosaurian known from a single almost complete specimen (BMNH R5488) from the Blue Lias Formation (Hettangian) of Harbury, Warwickshire, U.K. The Early Jurassic was an important time in the early evolution of the clade Plesiosauria and saw its diversification into two superfamilies, Plesiosauroidea and Pliosauroidea. As one of the earliest nearly complete, well-provenanced plesiosaurians known, M. tenuiceps is important for understanding early plesiosaurian evolution. Three new autapomorphies for Macroplata tenuiceps are presented, confirming the validity of this taxon: a triangular emargination in the posterior border of the coracoid; a triangular emargination in the posterior border of the ischium; and posterior convergence of the long axes of the posterior interpterygoid vacuities. Contrasting phylogenetic hypotheses of the position of M. tenuiceps and other Lower Jurassic plesiosaurians (some of which have not been treated scientifically since the 19th Century) highlights the need for revision of their anatomy and taxonomy for use in fine-grained species-level cladistic analysis.